1. Field of the Invention
This invention relates to methods for manufacturing devices of three-dimensional shapes using two-dimensional micro-electro-mechanical system (MEMS) techniques. The invention can be applied to materials with or without shape memory or superelastic properties, and has widespread applications, especially for fields and industries that have a demand for a high degree of miniaturization of devices and equipment.
2. Description of the Related Art
In recent years many medical devices have been introduced that incorporate shape memory and superelastic materials, especially titanium nickel alloys often referred to as TiNi or Nitinol. Two principal products are guidewires for catheters and stents used in the treatment of vascular disease. Use of Nitinol in stents fabricated from small diameter Nitinol tubing has grown rapidly.
Fabrication of stents includes the operations of laser cutting, expansion, heat treatment, and electropolishing. These processes are labor intensive and costly. Stents made from tubing are generally lacking in flexibility required to treat small blood vessels in the brain that must be accessed intralumenally through tortuous distal paths in the carotid artery.
Accordingly there is a need for improved devices to be used in the intracranial vasculature. The present invention provides an alternative method of fabricating tools for endolumenal use that result in devices that are small, flexible, inexpensive to make, smooth-surfaced, chemically resistant, and possess superelastic and shape memory properties. These devices include stents, filters, blood clot retrievers, aneurysm closures, and anastomosis devices for use in conjunction with blood vessel transplants.
Thin TiNi film has desirable characteristics for fabricating these devices, especially because it can be rolled, folded, or otherwise compressed for insertion through micro-catheters.
Production of complete systems for minimally invasive vascular treatment involves joining of components by welding, brazing, soldering, and adhesives. Superior performance can be achieved if the number of such attachments is minimized. Additionally, welding of thin film (micrometers thick) presents novel problems. It is desirable to make the device all in one piece to achieve maximum flexibility, strength, and minimal thrombo-genicity.
The most common method of producing thin metal films is by vacuum sputtering, generally onto a planar substrate. Sputtering onto three-dimensional substrates can be accomplished by rotation of the substrate in or near the plasma, and by cylindrical sputtering. While it has been shown that it is possible to sputter three-dimensional shapes, it is also known that the material thus produced by this method is not of the highest quality, and the methods do not lend themselves to production of large numbers of devices at low cost.
In cylindrical sputtering it is difficult to achieve the correct chemical composition. For intravascular use, the transition temperature should be below body temperature, 36.6xc2x0 C., to take advantage of superelasticity.
In three-dimensional deposition it is difficult to achieve good crystal structure of the deposited thin film alloy. This requires shielding to produce line-of-sight normal deposition, otherwise columnar structure appears with poor intra-crystalline and inter-crystalline adhesion. Brittleness results.
It is also difficult to remove the three-dimensional structure from the substrate. Etchants must be extremely selective, and must not interact with the thin film material, generally TiNi or TiNi-based.
Miniature devices made of free-standing thin film shape memory alloys such as Nitinol have potential applications in medicine, particularly in minimally invasive surgery of the vasculature. For a majority of endolumenal applications it is essential that these devices have three-dimensional shapes, i.e. cones, cylinders, and hemispheres, to take advantage of superelastic and shape memory properties. In applications relating to tissue engineering, these three-dimensional shapes of Nitinol thin film can be used as a base structural material or scaffold on which to grow artificial tissue cells. For example, tissue grown on a thin film cylinder will produce an artificial blood vessel in a tubular shape.
The most practical method of creating thin films of shape memory alloy is by vacuum sputtering. Sputtering onto three-dimensional substrates is wasteful, slow, and subject to difficulties not encountered in planar deposition.
The need has therefore been recognized for medical device fabrication methods which obviate the foregoing and other limitations and disadvantages of prior art fabrication methods and devices of the type described. Despite the various fabrication methods and devices in the prior art, there has heretofore not been provided a suitable and attractive solution to these problems.
It is a general object of the invention to provide methods of manufacturing devices of three-dimensional shapes using two-dimensional micro-electro-mechanical techniques.
Another object is to provide methods to make multiple layered three-dimensional shapes with pockets created selectively between the layers, such as for endovascular, endolumenal, intracranial, and intraocular medical applications.
Another object is to provide devices made by methods of the type described.
The prior art limitations and limitations described above are overcome in the present invention by methods that comprise planar material deposition that has advantages including high deposition rates, improved control of composition, and use of large substrates to make batches of devices rather than individual devices. This leads to a practical manufacturing method for large volume production.
The methods of the invention comprise the removal of material by chemical means that, combined with planar sputter deposition of multiple layers, photolithography, and heat treatment, enables the fabrication of hollow shapes having thin film surfaces of sufficiently small size for use in medicine, including endovascular, endolumenal, intracranial, and intraocular applications.